The Main Sensor Types and What Each One Measures
Selecting sensors requires matching physical transduction principles to measurands while accounting for thermal drift, noise floor, and fab availability.
Metal Oxide Gas Sensors
Chemiresistive Technology
Sensing elements composed of semiconducting materials change their electrical conductivity when gas molecules adsorb onto the surface. These metal oxide gas sensors typically operate at high temperatures to facilitate the necessary chemical reactions. The most common material used is tin dioxide, which reacts with reducing or oxidizing gases.
A small heater integrated into the sensor package maintains the required operating temperature.
Detection Mechanism
Oxygen ions on the surface of the heated semiconductor react with target molecules to alter the concentration of charge carriers. In metal oxide gas sensors, the resistance increases or decreases depending on whether the gas is reducing like methane or oxidizing like nitrogen dioxide. This change is logarithmic over several orders of magnitude of concentration.
The response time depends on the rate of gas diffusion and the surface reaction kinetics.
Power Requirement
Continuous operation of the heating element results in higher power consumption compared to electrochemical alternatives. Because metal oxide gas sensors must stay hot to remain active, they are often used in fixed installations rather than battery-powered portables. Recent developments in micro-electro-mechanical systems have reduced the thermal mass of the heaters.
This allows for pulse-mode operation to save energy.
Sensitivity Range
High sensitivity allows these devices to detect low parts per billion concentrations but they often lack the selectivity to distinguish between similar volatile organic compounds.